US 20070258537 A1 Abstract In an embodiment of the method, at least one signal-to-interference-and-noise ratio (SINR) for each antenna configuration in a set of transmission antenna configurations is determined based on an estimated channel characteristic. At least one received signal characteristic is determined for each antenna configuration in the set of antenna configurations based on the determined signal-to-interference-and-noise ratios. One of the antenna configurations in the set of antenna configurations is selected based on the determined received signal characteristics.
Claims(22) 1. A method of determining at least one transmit mode parameter for a multiple-input-multiple-output (MI MO) system, comprising:
determining at least one signal-to-interference-and-noise ratio (SINR) for each antenna configuration in a set of transmission antenna configurations based on an estimated channel characteristic; determining at least one received signal characteristic for each antenna configuration in the set of antenna configurations based on the determined signal-to-interference-and-noise ratios; and selecting one of the antenna configurations in the set of antenna configurations based on the determined received signal characteristics. 2. The method of the determining at least one SINR step includes determining an effective SINR for each antennas configuration; and the determining at least one received signal characteristic step determines at least one received signal characteristic for each effective SINR. 3. The method of 4. The method of 5. The method of 6. The method of determining a transmission throughput associated with each antenna configuration based on the received signal characteristic; and selecting the antenna configuration associated with a highest determined throughput. 7. The method of 8. The method of applying the effective SINRs to look-up tables to obtain the received signal characteristics, each look-up table associated with a different possible combination of additional transmission mode parameter values. 9. The method of 10. The method of determining figures of merit based on the block error rates and the additional transmission mode parameter values associated with the look-up tables, each figure of merit associated with one of the antenna configurations and one of the combinations of additional transmission mode parameter values; and selecting the antenna configuration associated with the block error rate producing a best figure of merit; and selecting the additional transmission mode parameter values associated with the best figure of merit. 11. The method of 12. The method of 13. The method of filtering, for each antenna configuration, the block error rates produced from a look-up table based on the determined effective SINRs for the antenna configuration; and wherein the determining the figures of merit step determines the figures of merit based on the filtered block error rates and the additional transmission mode parameter values. 14. The method of 15. The method of obtaining at least one received signal characteristic from look-up table using the determined SINRs, each look-up table associated with a different possible combination of additional transmission mode parameter values. 16. The method of 17. The method of determining figures of merit based on the block error rates and the additional transmission mode parameter values associated with the look-up tables, each figure of merit associated with one of the antenna configurations and one of the combinations of additional transmission mode parameter values; and selecting the antenna configuration associated with the block error rate producing a best figure of merit; and selecting the additional transmission mode parameter values associated with the best figure of merit. 18. The method of 19. The method of 20. The method of filtering, for each antenna configuration, the block error rates produced from a look-up table based on the determined SINRs for the antenna configuration; and wherein the determining the figures of merit step determines the figures of merit based on the filtered block error rates and the additional transmission mode parameter values. 21. The method of 22. The method of Description This invention was made with Government support under Contract W911NF-04-C-0025 awarded by the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention. The present invention relates to MIMO (multiple-input multiple-output) devices and communications, and more particularly to a method of adapting the number of transmitting antennas in a MIMO wireless link. MIMO represents an advance in wireless communication. MIMO employs multiple antennas at the transmitting and receiving ends of a wireless link to improve the data transmission rate while holding radio bandwidth and power constant. A MIMO transmitter transmits an outgoing signal using multiple antennas by demultiplexing the outgoing signal into multiple sub-signals and transmitting the sub-signals from separate antennas. MIMO exploits the multiple signal propagation paths to increase throughput and reduce bit error rates. Each sub-signal reflects off the local environment along its associated signal propagation paths. The spatial richness of the local environment is a function of the uniqueness and distinctness among the different associated signal propagation paths. While multiple signal propagation paths cause interference and fading in conventional radios, MIMO uses these multiple signal propagation paths to carry more information than conventional radio transmissions. Using MIMO techniques it is possible to approximately, linearly increase the rate of transmission, depending on the richness of the local environment. In a traditional Frequency Division Duplex (FDD) system, the MIMO receiver must determine M and feed this back to the transmitter on a separate low rate channel, such as mode selection link In the situation where both the transmitter and receiver are stationary, the estimated channel characteristic of the MIMO system remains relatively stable, as does the optimal number of transmission antennas. However, where the transmitter, receiver, or objects in the environment are mobile, the actual channel characteristics of the connection and the spatial richness of the environment can change in response to movement. As the spatial richness of the environment changes, it becomes beneficial to vary the number of active antennas in the MIMO system to optimize the throughput of the wireless transmission. Varying the number of antennas in the MIMO system can offer various benefits including improved transmission rates, reduced interference among sub-signals, lower latency, and reduced power consumption. For example, as the spatial richness in an environment increases, it may be beneficial to harness the increased variation in the multi-path signal propagation by increasing the number of active antennas. Alternatively, as the spatial richness decreases it may be beneficial to reduce the number of active antennas to avoid potential interference due to the limited signal path variations, and reduce power consumption by using fewer active transmitting antennas, which would otherwise cause interference. Therefore, there exists a need for a method to dynamically alter the number of transmitting antennas in response to, for example, changes in spatial richness. In an embodiment of the method, at least one signal-to-interference-and-noise ratio (SINR) for each antenna configuration in a set of transmission antenna configurations is determined based on an estimated channel characteristic. At least one received signal characteristic is determined for each antenna configuration in the set of antenna configurations based on the determined signal-to-interference-and-noise ratios. One of the antenna configurations in the set of antenna configurations is selected based on the determined received signal characteristics. In one embodiment, the determining at least one SINR step includes determining an effective SINR for each antennas configuration. For example, the determining an effective SINR for each antenna configuration step determines each effective SINR based on SINRs associated with the transmission antennas in the antenna configuration. The determining at least one received signal characteristic step determines at least one received signal characteristic for each effective SINR. For example, the received signal characteristic may be block error rate. In one example embodiment, the determining at least one received signal characteristic step applies at least one of the effective SINRs to at least one look-up table to obtain a received signal characteristic. In another embodiment, the determining at least one received signal characteristic step determines a received signal characteristic for each antenna configuration and at least one additional transmission mode parameter based on the determined effective SINRs. For example, the determining at least one received signal characteristic step may include applying the effective SINRs to look-up tables to obtain the received signal characteristics, each look-up table associated with a different possible combination of additional mode parameter values. As stated above, the received signal characteristic may be block error rate. Here, the selecting step may include determining figures of merit based on the block error rates and the additional transmission mode parameter values associated with the look-up tables. Each figure of merit is associated with one of the antenna configurations and one of the combinations of additional transmission mode parameter values. The antenna configuration associated with the block error rate producing a best figure of merit is selected, and the additional transmission mode parameter values associated with the best figure of merit are selected. For example, the figure of merit may be throughput, and the additional transmission mode parameters may include at least one of encoding rate and modulation order. In one embodiment, the selecting step further includes filtering, for each antenna configuration, the block error rates produced from a look-up table based on the determined effective SINRs for the antenna configuration. The determining the figures of merit step determines the figures of merit based on the filtered block error rates and the additional transmission mode parameter values. The present invention will become more fully understood from the detailed description given herein and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limiting of the present invention and wherein: As will be described in detail below, the present invention provides embodiments of a transmitter, receiver and method for adapting the MIMO wireless link. In particular, the transmission mode may be adapted. The transmission mode may include an indication of the number of transmitting antennas to use during transmission; the choice of modulation order (e.g., QPSK, 16-QAM, etc.); the encoder rate; the number of CDMA codes (or, OFDM tones, etc); and/or etc. The present invention may be compatible with any desired method of signal modulation and coding. However, for the purposes of example only, the embodiments of the present invention will be described for a CDMA system using K orthogonal codes. Based on the mode request, an FEC encoder A demultiplexer A set of multipliers As will be appreciated, the transmitter includes as many code demultiplexers As shown in The pilot signal training unit As is known, the receiver arrangement A minimum mean square error (MMSE) solver Next using the channel characteristic for the transmitting antenna configuration of the current transmission mode, the MMSE solver The MMSE solver Based on the inputs, the effective SINRs, a LUT produces a block error rate (BLER) that would be expected for this transmission mode given the effective SINR. For each antenna configuration in the set, the output from each LUT is averaged or filtered over time to remove signal fading effects. For example, for the effective SINRs generated for the antenna configuration of four transmission antennas over time, the output from each respective LUT is averaged or filtered. For each of the filtered outputs from the LUTs The mode selector Next, the expressions upon which the MIMO link adaptation methodology is based will be described in detail. This will be followed by a description of an embodiment of the MIMO link adaptation methodology as shown in the flow chart of As will be understood from the description given above, the receiver arrangement Next, the relationship between the particular channel characteristics and the resulting signal-to-interference-plus-noise (SINR) ratio will be described. First, consider a flat fading channel. For reference, the SINR of a 1×1 system, conditioned on the channel response, is,
In a MIMO system, the received vector after despreading any of the K codes is,
There are two equivalent forms for the MMSE filter, the right-inverse and left-inverse forms. The right inverse form requires inversion of an N×N matrix, while the left inverse requires an M×M matrix. Where there are fewer transmitters, the second form is more computationally efficient. The filtered output for stream m is:
Now consider the frequency-selective case. Due to the resolvable multi-path, the codes are no longer orthogonal, and inter-code interference cannot be neglected. Starting with a 1×1 system, assume h is the 1×L channel impulse response vector. First, it is necessary to determine the SINR at the output of a correlator set to the l-th delay. Defining the k-th spreading code at time delay l as the vector, the total transmitted signal is,
Following the standard analysis, there will be a desired term, GA In the MIMO case, the SINR is determined at the despreader outputs, including contributions from the other antennas. The signal transmitted from the m-th antenna is,
The signal received at the n-th antenna has contributions from all M transmitters,
Despread the n-th received signal with the k-th code at the l-th delay, the calculation becomes:
One can see there is a desired term, and an interference plus noise term,
Using this description, we can see the composite model for the received signal of the kth code output from the receiver arrangement GA _{k},0 . . . ,0)^{2} ^{H} w _{m} ^{H }
I _{m} =w _{m} diag(GA _{k} , . . . ,GA _{k},0,GA _{k} . . . ,GA _{k})^{2} ^{H} w _{m} ^{H } (20)
N _{m}=w_{m}Dw_{m} ^{H }
Having laid the mathematical foundation for the method according to the present invention, the MIMO link adaptation methodology according to an embodiment of the present invention will now be described with reference to the flow chart of As shown, in step S In step S The MMSE solver To review, each antenna configuration in the set has an associated channel characteristic, spatial filter matrix and set of M SINR values. In step S The effective SINR values are sent to the mode selector Based on the inputs, the effective SINRs, a LUT produces a block error rate (BLER) that would be expected for this combination of transmission mode parameter values given the effective SINR. For each antenna configuration in the set, the output from each LUT is averaged or filtered over time to remove signal fading effects. For example, for the effective SINRs generated for the antenna configuration of four transmission antennas over time, the output from each respective LUT is averaged or filtered. For example, a simple averaging of the most recent 3, 5 or 7 BLER outputs from a LUT for an antenna configuration may be performed. For each of the filtered outputs from the LUTs As stated, in one embodiment, the figure of merit f is chosen to be the throughput, which is defined as follows:
As will be appreciated, if only the antenna configuration was being determined as the transmission mode, then equations (23) and (24) would be reduced to including only the BLER portion of the equations. While a majority of the analysis presented has been for the case of a CDMA system with a rake receiver arrangement, it will be appreciated that any linear receiver can be accommodated. For example, if a chip-level linear MMSE equalizer is used, it is only necessary to expand the set of delays in the weight vector to exceed the channel duration by a suitable factor. Similarly, the present invention may also be applied to an OFDM system. With some embodiments of the present invention having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications are intended to be included within the scope of the present invention. Referenced by
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